Nuclear quadrupolar resonance (NQR) is the response of a certain compound containing any quadrupolar nucleus to a high frequency pulse which is applied “on resonance”. It is specifically used for the detection of explosives and other forbidden substances which may be hidden within luggage and packages, which substances detection is currently difficult. The apparatus has two versions: one of them to be used for hand luggage, i.e. briefcases, bags, purses, etc., the other version to be used in bigger luggage pieces such as those usually transported in aircrafts holds.
Quadrupolar resonance technique is absolutely harmless to environment, luggage and humans, as it involves luggage radiation with radio waves of very long wavelength or of low frequency -on the order of some MHz-, along with the simultaneous application of magnetic field pulses amounting to some tens of Gauss, even fewer than those applied in the known magnetic resonance imaging (MRI). This technique application is direct, and previous conditioning of objects to be inspected is not required. This method implies very quick routine inspections. Typically the verification for explosives in luggage or transported packages takes one or two seconds without neither opening same nor contacting them with any mechanical and/or palpation tool. No ionizing radiations are used, thus avoiding any danger to luggage or individuals. Detection is univocal and each apparatus is fully computerized, this fact allowing an easy operation which dispenses specialized personnel who should have to make subjective decisions.
Nuclear quadrupolar resonance (NQR) is an spectroscopic technique of frequent use in chemical and physical analyses of non-metal materials. Response generated by the nuclear quadrupolar resonance (NQR) is characteristic of magnetic and electric properties of resonant nuclei. The nuclear quadrupolar resonance (NQR) phenomenon may only take place with certain atoms (which nuclei exhibit nonnull quadrupolar moment, namely spin I>½), and is frequently easily observed when same are part of crystalline or amorphous materials. Thus, for example, all those explosives containing chlorine and/or nitrogen are potentially detectable by means of this technique.
Nitrogen nuclear quadrupolar resonance (NQR) signals in RDX and other explosives (e.g., see: V. S. Grechishkin, “NQR device for detecting plastic explosives Mines and Drugs”, Applied Physics, Vol. A55, pp. 505-507 (1992)) have already been observed with sensitivity enough to form the base of a detector capable to be used in order to investigate traveling bags and closed mail, personal carriers, etc. The resonance phenomenon in nitrogen substances is mainly observed in the high frequencies range, i.e. explosive detection is accomplished through radio waves, conveniently conditioned by means of special electronic devices. Each chemical compound the explosive substance is composed of may possess one or more resonance frequencies which are generally unique and help to distinguish same from other compounds present in nature.
Electric and magnetic properties of atomic nuclei produce the nuclear quadrupolar resonance (NQR) phenomenon. Nuclei with spherically non-symmetrical electric charge possess a quadrupolar electric moment. Other nuclear property consists of the possession of a magnetic moment, also known as nuclear spin. Nuclear quadrupolar resonance originates upon the interaction between the nucleus electric quadrupolar moment and the [gradient of] electric field originated from the electric charges adjoining the nucleus.
Graphically, albeit not in a rigorous manner: it can be said that when a quadrupolar nucleus experiments an electric field gradient resulting from an atomic environment, this occurs as if different portions of the nucleus were experiencing a torque making them to precess (rotate) around the maximum variation axis direction (gradient) of the electric field in the quadrupolar nucleus position. This precession movement “drags” the nuclear magnetic moment. Should the sample be temporarily subjected to an oscillating magnetic field, “tuned” with this precession, the nuclear magnetic moment orientation as regards the electric field gradient direction could be modified. Such oscillating electric field is simply achieved by placing the sample or object to be detected at the vicinity of an antenna which is connected to a radio frequency generator during a convenient period of time (typically on the order of microseconds) known as “radio frequency pulse”. Upon the termination of the pulse, the magnetization of the sample, which precesses with the quadrupolar resonance frequency, produces a detectable signal known as “free induction decay signal”, usually named “FID”.
The above mentioned precession frequency depends on two parameters:                firstly it is proportional to the quadrupolar moment P of the nucleus, which is in turn related to the internal electric charge distribution of said quadrupolar nucleus. P parameter is zero in those cases in which the charge distribution of the nucleus has a spherical symmetry, positive when the charge distribution is elongated along the main axis, and negative when it is flat relative to said axis. Symmetry properties of the nucleus require that a necessary condition for the nucleus P to be different from zero is that the spin quantic number (or magnetic quantic number) be higher than one half: I>½; and        secondly, frequency is controlled by the electric field's main component, q.        
For example, in the case of a group of spin I= 3/2 nuclei, resonance frequency when no external magnetic field is present is given by: v=e2qP/4h, h being the Planck's constant and e the electron charge. In the case of nuclei with spin I=1, up to three resonance frequencies can be observed, namely: v+/−=(3e2qP/4h) (1+/−η/3)yvo=(e2qP/2h)η, wherein η is termed electric field gradient asymmetry parameter.
The purpose of these definitions is to show that the resonance frequency value, which may be measured with high accuracy in any nuclear quadrupolar resonance (NQR) experiment, is a characteristic magnitude of the molecule bearing the resonant nucleus, such as a “fingerprint”. There exist in nature many different quadrupolar nuclei. Those commonly present in explosives are nitrogen, chlorine, sodium, potassium, etc. All of these nuclei are detected by routine in nuclear quadrupolar resonance (NQR) spectrometers used in scientific research, and the same happens in the case of explosives. For example, it is possible to inspect the presence of different explosives by adjusting the detector to the characteristic frequence(s) of said molecule, which must be previously well known.
Many devices which use pure quadrupolar resonance have been invented in order to detect different forbidden compounds or substances. As used herein, “pure” means the non-inclusion of an external magnetic field, also known as “Zeeman magnetic field”.
Generally, compounds are crystalline solids characterized in that the free induction decay signal (FID) and the shape of the nuclear resonance line of a group of spins A nuclei are mainly defined by the coupling of their magnetic moments to the magnetic moments of another group of different spins B nuclei. In these cases coupling within the same spins A may be neglected, and consequently the loss of coherence regarding the precession phase of spins A is due to fluctuations in local magnetic fields generated by spins B, which occupy neighboring positions in the crystalline net or in the molecule itself. A previous work by Herzog and Hahn (B. Herzog and E. L. Hahn, Phys. Rev. 103, 148 (1956)), demonstrates that by applying a weak magnetic field H0 (on the order of some Gauss) and by continually irradiating protons in resonance condition with an oscillating H2 magnetic field, said coupling can be destroyed. As the decaying time of cross coherence of quadrupolar nuclei is almost exclusively due to fluctuations of local fields produced by the protons, protons double radiation averages said fields to zero, producing a remarkable increase of the decay time of magnetization of the group of spins A nuclei.
Physical explanation is that the line width of spins A suffers a marked narrowing when the externally forced reorientation speed of spins B is sufficiently high so as to cause the reduction to small values of the mean value of the local field produced in the spins A nuclei group. This average is similar to the effect known as “motional narrowing” in liquids, to the “line narrowing” obtained upon the mechanical rotation of liquid samples in a non-uniform external magnetic field, and also to the “spinning” or mechanical rotation of solids, to narrow the nuclear magnetic resonance (NMR), broadened by local magnetic fields. In order for the mechanical rotation to be effective, rotation speed must exceed the broadening of Larmor frequencies produced by the lack of field homogeneity. Similarly, line narrowing due to the double resonance in solids requires that the speed of reorientation of spins B to exceed the minimum broadening in Larmor frequencies of spins A, existing upon the lack of double resonance. With the experiment of double resonance, decay time of the envelope of the spin echoes of spins A, known as T2, increases or decreases depending on a combination of effects:                1) internal couplings among spins A (homonuclear coupling); and        2) coupling between spins A and B (heteronuclear coupling).        
For an oscillating magnetic field H2 with adequate intensity, which also accomplishes the resonance condition for spins B in weak magnetic field H0, the decay time as regards the envelope of echoes T2 is extended up to the theoretical maximum limit imposed by the time extension of longitudinal decay T1, or otherwise by the time extension of decay T2 of spins A, whichever the lower.